Power plants built during the past decade in the US were mostly natural-gas-fired gas turbine combined cycle (GTCC) plants. Unfortunately, the operating capacity factor of these units is less than 25 percent due to the persistent high cost of natural gas. It is unlikely that the price of natural gas will return to the $3-$4 per million Btu level that served as the basis for the economics of these plants.
The use of syngas from lower-cost fuels such as coal to refuel these potentially stranded plants has been considered by many plant owners, but the large capital expenditure associated with the gasification facilities has been a deterrent. In addition, the uncertainties regarding carbon dioxide (CO2) emissions requirements have led to delay or cancellation of many recent coal projects.
This invention provides a design for a lower-cost, thermally efficient, and standalone coal gasification plant for refueling natural gas combined cycle (NGCC) plants. The lower plant cost is achieved through the combination of two design features: a) use of a low-cost, quench-type gasifier rather than a gasifier with syngas coolers, and b) use of a low pressure (150 psi) steam turbine augmented by a binary cycle. This power recovery approach generates electricity from the low-level energy available downstream of the gasifier with an efficiency competitive with that of current coal-fired power plants.
In a coal gasification plant equipped with syngas cooler, about 20-25 percent of energy in coal is consumed during the exothermic gasification reaction, and it is partially recovered in the form of high pressure steam at about 1800 psig. Implementing a full thermal integration between a gasification plant and a combined cycle plant to refuel an existing GTCC plant is generally not practical due to two reasons: 1) relatively high costs of full recovery coal gasification plant equipped with syngas cooler, and 2) the existing steam turbine and heat recovery steam generator (HRSG) are unlikely to have excess capacity to accept the large amounts of saturated steam. Possible exceptions are NGCCs with high duct-firing capability.
This invention employs a quench gasifier to generate syngas. Based on published cost information, the cost of a quench gasifier is substantially lower than that of a radiant quench syngas cooler. For an integrated gasification combined cycle (IGCC) plant without carbon capture, the quench-gasifier-based plant is 11-14 percent lower in overall capital cost than the syngas-cooler-equipped gasification plant. In addition, the reliability of quench gasifiers has been commercially proven. Commercial coal gasification plant has reported that the quench gasifiers have on-stream reliability factors of 94-97 percent which is substantially higher that of the syngas-cooler-equipped gasifiers which is generally less than 85 percent.
This invention provides a method to refuel existing NGCC as a non-integrated gasification combined cycle (nIGCC) plant. The unique heat recovery scheme developed for this invention allows the NGCC to increase net power plant output with good thermal efficiency relative to conventional pulverized coal fired power plant. A NGCC plant rated at 250-260 MW net output is likely to produce up to 275 MWe net output. This added output is due to the increase in fuel mass flow as compared to that of natural gas. An overall thermal efficiency of 36%+ is competitive to most existing coal fired power plants.
If carbon capture is desired, a water shift converter can be placed upstream of the low pressure steam generator. Shift-converter requires the water to CO molar ratio to be about 2-2.5 to 1. Because water moisture is added to the syngas in the quench gasifier and the downstream water scrubber, additional steam injection to the syngas is not required for the shift converters. The syngas from a quench gasifier is considered a shift conversion ready syngas, and this makes the process more carbon capture ready than that of other processes.
The superheater (HRSG 7) can be supplementally fired with clean syngas produced internally to augment steam production to match the need for auxiliary power in the gasification plant. The amount of supplemental firing can be adjusted to automatically match an auxiliary load requirement for the coal conversion plant so that the entire coal-to-gas conversion facility is self sufficient in power supply.